Calculation of reswitching surge current in an induction motor

The paper investigates the transient phenomena in an induction motor after reswitching. It is assumed that prior to reswitching a current flows in the rotor winding. The maximum peak value of the transient current does not occur if the voltage induced by the rotor current and the supply voltage are in phase at the instant of reswitching. The conditions for both the severest and the most advantageous reswitching transients during the first period after reswitching are examined. By using special, but now known, devices built into autoreclose circuit breakers, it is possible to perform reswitching under the most advantageous conditions. This leads to a considerable reduction of stator surge current. In the case of an 8200 kW motor, mentioned in the paper, the surge current at 0.7 synchronous rev/min can be limited to 2 p.u. instead of 18 p.u., which occurs in the most severe case. This reduces electrodynamic forces acting on stator end windings. Since the forces are proportional to the square of the reswitching current, their reduction (1:81) in the above mentioned example prevents end-winding insulation breakdowns caused by dangerous mechanical vibration during reswitching.